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Date: Wed, 21 Jul 2021 09:56:38 -0700 (PDT)
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Subject: Re: Gnat bug or mistaken program?
From: Shark8 <onewingedshark@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:62420
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On Wednesday, July 21, 2021 at 2:29:22 AM UTC-6, Jeffrey R. Carter wrote:
> > Thank you. So it is a visibility problem.
> "Understanding visibility is the key to understanding Ada." -- /Ada Distilled/ 
> 
> Recommending the use package clause as a solution to a misunderstanding of 
> visibility is a disservice to a beginning user. Widespread application of use 
> pkg clauses is a crutch to avoid understanding visibility. I recommend that 
> those who do not understand visibility avoid the use clause altogether, as this 
> forces them to learn about visibility. When one understands visibility, one can 
> then make reasoned decisions about whether and when to add use clauses. 
> 
> In decreasing order of specificity, the ways to call an operation in a pkg are 
> 
> * Use the full name: Ada.Numerics.Elementary_Functions."**" (10.0, Logs) 
> This calls the operation once without changing its visibility 
> * Rename the operation: 
> function "**" (Left : Float; Right : Float) return Float renames 
> Ada.Numerics.Elementary_Functions."**"; 
> This makes the specific operation visible 
> * Use type: this makes all operators of the type visible (not applicable 
> in this case) 
> * Use all type: this makes all operations of the type visible (not 
> applicable in this case) 
> * Use package: this makes everything in the package visible 
> 
> Use pkg is clearly overkill for this case, and overuse of it can have negative 
> consequences. 
> 
> -- 
> Jeff Carter 
> "Choose a data representation that 
> makes the program simple." 
> Elements of Programming Style 
> 188

This is excellent advice and an good list, though the last element should be two or three:
* Local, via declare-region/-scope; and
* ["local" via usage inside the spec, (perhaps in the public portion, perhaps in the private); and]
* Global (top of the file context-clauses).